The present invention relates to color proofing foils and refers more particularly to methods and materials for making color proofing foils of dark colors of improved developability from light sensitive materials.
U.S. Pat. No. 3,326,682 teaches that positive working color proofing foils can be prepared by coating o-quinone diazide photosensitizers and colorants, with or without resins and other additives, upon a transparent support, preferably polyethylene glycol terephthalate. In the practice of the above patent, the contents of which are explicitly made part of this application by reference, excellent color separation overlay proofing materials have been made from positive silver halide color separations used as exposure masks upon the appropriate colored proofing foil. After exposure and development in dilute aqueous alkali, the foils may be viewed individually and then placed one above the other on top of a white reflecting support to give a preview of the print which is to be made from plates from the aforementioned color separation silver halide positives.
A black coated foil is customarily used as one of the layers to add depth of color beyond that which would be obtained from a three color separation restricted solely to cyan, magenta and yellow. It has been found by experience that when a black dye or pigment is added to the foil coating composition, a great increase in required exposure results due to the absorption of light by the black dye or pigment. Dark colored foils, either black or other dark colors, similarly require undesirably increased exposure.
In the case of the black in positive systems, one approach to reduced exposure has been to reduce the level of dye or black pigment. This has the undesirable effect of giving a medium gray or light gray rather than deep, rich, black which would be preferred. These considerations also apply to other dark colors, which may be used apart from multicolor proofing processes in the graphic arts.
Still another approach to overcoming the exposure increase resulting from the full quantity of black dye or pigment is disclosed in U.S. Pat. No. 4,154,613. According to this invention, it was found that an overall exposure to light prior to or after imagewise exposure results in a highly desirable reduction of required imagewise exposure even though the dye or pigment level in the coating was at the desired level required for full, deep, rich colors. However, special equipment and an added process step is required.
A recent development in the technology of color proofing has been the introduction of automatic exposure and development machinery. With the use of such equipment, it is desirable that foils of all colors either deeply or lightly tinted have the same exposure speed and development characteristics, in order to avoid individual manual attention in processing. With the exception of the aforementioned technique of overall pre or postexposure, there has existed no convenient method of adjusting the exposure speed of the various differently colored proofing foils.
Therefore, one object of the present invention is to provide methods and materials for making color proofing foils which overcome the disadvantages of the prior art.
Another object is to provide novel light sensitive materials for making color proofing foils of increased light sensitivity.
Another object is to provide novel light sensitive materials for making color proofing foils in which exposure speed can be varied independently of the color selected.
Still another object is to provide foils of improved developability in which the required development time may be adjusted so that it is the same for all colors.